When you create a Studio project by using the Create an audiobook option, and import a document that incudes chapters, the Chapters sidebar will be enabled automatically, and you can add a new chapter using the + button at the top of the Chapters sidebar.
If the Chapters sidebar is not already visible, you can toggle visibility by clicking Chapters.
If you used Start from scratch to create your project, or your project didn't originally include chapters, and you now want to add chapters, you need to enable chapters from within Project settings.
You can access the Project settings by clicking on the three small lines next to the “Export” button in the upper right corner. The Enable chapters option can be found in the General settings.
Once you've enabled chapters for your project, you can click + Chapter to add a new chapter to your project. Once you've added one chapter, the Chapters sidebar will be enabled, and you can use the + button to add additional chapters.